1. Technical Field
The present invention relates to an exhaust gas purification system abnormality diagnosing device and abnormality diagnosing method and an exhaust gas purification system. The present invention particularly relates to an exhaust gas purification system abnormality diagnosing device and abnormality diagnosing method and to an exhaust gas purification system that use ammonia to reduce NOx (nitrogen oxides) in exhaust gas.
2. Related Art
Nitrogen oxides (hereinafter called “NOx”) are often included in exhaust gas emitted from internal combustion engines such as diesel engines. As an exhaust gas purification system for reducing the NOx and purifying the exhaust gas, there is known an exhaust gas purification system that injects a reducing agent derived from ammonia on an upstream side of a selective reduction catalyst disposed in an exhaust gas passageway, causes the selective reduction catalyst to adsorb the ammonia, causes the NOx in the exhaust gas flowing into the selective reduction catalyst to reaction with the ammonia, and purifies the exhaust gas.
In this type of exhaust gas purification system, sometimes a sensor for detecting the NOx concentration on the downstream side of the selective reduction catalyst is disposed and injection control of the reducing agent is performed such that a sensor value of this sensor becomes less than a predetermined value. However, sensors for detecting the NOx concentration that are disposed in this type of exhaust gas purification system often have the characteristic that they respond also to the ammonia in addition to the NOx. For that reason, in an exhaust gas purification system that uses a reducing agent capable of generating ammonia, the sensor responds and the sensor value rises even when the ammonia has flowed out to the downstream side of the selective reduction catalyst.
Ordinarily, the sensor value of the NOx sensor is maintained less than an allowable value because the target injection quantity of the reducing agent is obtained by computation in response to the quantity of the NOx emitted from the internal combustion engine. However, when degradation of the entire exhaust gas purification system occurs, sometimes the deviation of the actual injection quantity with respect to the target injection quantity increases, the catalyst efficiency drops, and the NOx concentration or the ammonia concentration on the downstream side of the selective reduction catalyst exceeds the allowable value.
The content of measures and control that should be performed thereafter differs between a state in which the NOx concentration on the downstream side of the selective reduction catalyst is rising and a state in which the ammonia concentration on the downstream side of the selective reduction catalyst is rising. For that reason, it becomes necessary to identify which state is causing the rise in the sensor value. Therefore, there have been disclosed methods of determining, in an exhaust gas purification system that causes NOx to react with ammonia to purify the exhaust gas, the abnormal state of the exhaust gas purification system on the basis of the sensor value of the NOx sensor.
For example, there has been disclosed a method by which the added quantity of the ammonia is increased when the actual NOx purification efficiency derived on the basis of the sensor value of the NOx sensor is equal to or less than a target NOx purification efficiency, it is determined that the quantity of the ammonia is insufficient when the actual NOx purification efficiency after the correction exceeds the actual NOx purification efficiency before the correction, and it is determined that the increased added ammonia is not acting on NOx purification—that is, that there is the potential for ammonia slip—when the actual NOx purification efficiency after the correction is equal to or less than the actual NOx purification efficiency before the correction (e.g., see JP-A-2003-293743).
Further, there has also been disclosed a method by which the reducing agent injection quantity is decreased when a difference (NOx concentration deviation) between the actual NOx concentration obtained by the NOx sensor and a proper NOx concentration that has been set beforehand has become greater than a predetermined value, it is determined that there is NOx slip when the NOx concentration deviation after decreasing the reducing agent injection quantity has become larger than the NOx concentration deviation before the decrease, and it is determined that there is ammonia slip when the NOx concentration deviation after decreasing the reducing agent injection quantity has become smaller than the NOx concentration deviation before the decrease (e.g., see JP-A-2008-157136).
However, the method described in JP-A-2003-293743 is one by which the reducing agent injection quantity is forcibly increased and the abnormal state of the exhaust gas purification system is determined by the change in the sensor value thereafter. For that reason, when the ammonia concentration on the downstream side of the selective reduction catalyst (hereafter called “the downstream-side ammonia concentration”) has risen, there is the fear that the downstream-side ammonia concentration will further rise by implementing the method described in JP-A-2003-293743.
Further, the method described in JP-A-2008-157136 is one by which the reducing agent injection quantity is forcibly decreased and the abnormal state of the exhaust gas purification system is determined by the change in the sensor value thereafter. For that reason, when the NOx concentration on the downstream side of the selective reduction catalyst (hereafter called “the downstream-side NOx concentration”) has risen, there is the fear that the downstream-side NOx concentration will further rise by implementing the method described in JP-A-2008-157136.
Moreover, in addition to a rise in the downstream-side NOx concentration or the downstream-side ammonia concentration, an abnormality in the NOx sensor where the sensor value of the NOx sensor becomes large with respect to the actual concentration is also conceivable as causing the abnormal state of the exhaust gas purification system that appears when there is a rise in the sensor value of the NOx sensor. Even if this abnormal state of the NOx sensor is occurring, it cannot be distinguished from a rise in the downstream-side NOx concentration or a rise in the downstream-side ammonia concentration.
Therefore, the inventor of the present invention earnestly endeavored to discover that, focusing on the fact that a selective reduction catalyst has the property that the quantity of the ammonia it is capable of adsorbing increases as the pressure of a gas (exhaust gas) becomes greater, this problem can be solved by raising the exhaust gas pressure with a passageway area limiting device when a gas concentration that is detected has become equal to or greater than a predetermined start-of-diagnosis threshold value or when a difference obtained by subtracting from the gas concentration a downstream-side NOx concentration estimated by computation has become equal to or greater than a predetermined start-of-diagnosis threshold value and determining the abnormal state on the basis of the gas concentration or the difference thereafter, and thus the inventor completed the present invention. That is, it is an object of the present invention to provide an exhaust gas purification system abnormality diagnosing device and abnormality diagnosing method and an exhaust gas purification system that can determine the abnormal state of an exhaust gas purification system without having to significantly increase the downstream-side NOx concentration or the downstream-side ammonia concentration.